It depends on how big the aro is. Do you know how big it will be and do you have an Idea of what species you might get? Silvers may be the most common and cheapest but they can be a hassle to house being that they grow so large.
TheRealMacDaddy;3070000; said:Much of what has already been said is very pertinent, but I'll add my 2 Abes.
Depth of the tank for an Arowana is the least concern, length and width are more important. Now I'm not advocating something 6 inches deep understand, but 18 inches is plenty even for a full size adult. But the tank needs to have enough length and width that the fish can turn without backing up or you risk gill roll.
So longer and wider is better than long and deep and skinny.
Good Luck with your aquisition whether it's a Black or a Silver.
Bderick67;3073927; said:While I will agree tank depth is not as important, it still should be given consideration. This is especially true if there are going to be tank mates. My 27" silvers usually cruise about 3~4" below the surface and they are about 5" tall. So I would think dedicating the top 10 to 12" of a tank to the silvers is needed. In an 18" tall tank most the middle dwelling fish would be into the aro's space. IMO I wouldn't go less then 24", myself I decide to go 30" on the new tank.